Thermal Gun. My unit of dead. To do any science it ideally will be accurate to within 0.5 degrees F within the range you expect leg temperature to be. Look in the electrical / tools dept of Lowes or similar stores. Price of minimal unit I would use was $49.99. You might also be able to find...
A variant on this where you might see a departure between feet and environmental temperatures. Measure the temperature of a chicken that is relaxed. Next have the bird exert itself in a sustained manner where it begins to pant. Then measure temperature again.
Chickens I have are bred for...
Consider contacting the local fire inspector to see if they have one you might be able to bring a chicken to. My infrared camera sees wave lengths that are too short.
I use something like shown below to measure where heat is lost from large fish tanks. Might do what you need although repeated measures required to get an average...
I am pretty certain temperature varies with temperature and core temperature of the bird. It does not normally cool to freezing and feels a little warm even when a bird is walking on snow. Otherwise lower ambient temperature mean cooler feet. When a bird is exerting itself or otherwise stress...